Story:
Agatha looked critically at the blackboard outside her lecture theatre. Someone had been meddling! Fortunately, there hadn't been anything vital on it, but she'd have to nip this kind of behaviour in the bud before it became common practice.

However, it was actually rather interesting - it wasn't some foolish thing, but rather a long timeline - covering the whole of known history, with a few major eras notated on it. Indeed, there was even a note that it was not to scale - which was true. If it had been it would've stretched around the entire university, and been pretty dull to look at, for the most part.

Babylon, Egypt, the Greeks, the Romans- so many major civilisations were notated on the line, and yet several others worthy of note were not included, to Agatha's puzzlement.

Then she peered at the modern era, examining the tiny text carefully. 'RE:IS Advent Challenge set' with a tiny asterisk was right next to another marker: 'Challenge 21: History of Exploration Challenge set'.

Puzzle: I've been around since the dawn of time itself and I'm used every day by millions. However, to find me by name in the game, only one person can direct you. What am I?

This puzzle has been solved by the community but the Grand Challenge Clue (half) remains locked. This may be unlocked with lifelines or deadline extensions before the end of the Challenge.

Solution:Mathematics

Highlight above to reveal the solution.

A complete submission will include:

Your answer for the puzzle in text.

The image shows the in-game solution text solution to the puzzle.

You may use either the base game or More To Explore for this puzzle.

Bonus point: If you get the right answer and find a second person who can direct you to the correct answer (thus proving me wrong) you get a bonus point. I'm relatively sure there's only one person though.

Additional Secret: For a major clue towards one of the grand challenge rules, the community must submit at least six different, correct solutions and include a picture of their resources, one of which must be at a value lower than 5. This image must be taken while the crew is in London between expeditions.

Deadline: Wednesday 28th December 12:00 noon CET

Solution: (Unknown)

Spoilers: You may wish to use spoilers so that other players can choose if they wish to know the answer while the puzzle is up or not. To do so, copy this: